In some cases a flower color is the result of a nonstable mutation so it may revert in time to its more native color. Flowers will occasionally change color according to any soil pH changes, which it will do gradually over time, especially if lime has been applied to raise it in the past. Easter lilies prefer a pH of 6.0 to 7.5 so perhaps that has something to do with it. To be honest, I do not know why your lily has changed, either.